Transistors can be employed as switches in electrical circuits, in particular, power MOSFETs can be employed as switches in a load circuit for coupling a load to a source. The transistor may be either switched on, i.e., to its conducting state, thus allowing a current flow through the load circuit, or the transistor may be switched off, i.e., switched to non-conducting.
When switching the transistor on, i.e., switching the transistor to conducting, the electrical load is coupled to the voltage or current source allowing a current to flow through the circuit, i.e., through the transistor and the load. When switching a transistor off, i.e., switching the transistor from conducting to non-conducting, the transistor will disconnect the load circuit from the source, such that the transistor will stop a current flow in the load circuit. However, if the load circuit comprises an inductor or a capacitor, the current flow in the load circuit will not stop immediately when switching the transistor off. If the circuit comprises an inductor, then this will discharge its stored energy, such that the current flowing in the load circuit and through the transistor decreases with time until the inductor is fully discharged.
The discharge current flowing through the MOSFET in combination with the voltage drop across the MOSFET during the switch-off process, i.e., while the complex load discharges, heats the MOSFET. In particular, the pn-junction within the MOSFET is heated by the energy absorbed in the MOSFET. This heating may destroy the MOSFET as the semiconductor material may become intrinsically conductive when heated above a threshold temperature, such that the MOSFET may not be controllable any more. Accordingly the energy absorbed by the MOSFET should be limited such that the MOSFET can handle the switch-off process without being damaged.
A strong discharge current furthermore affects a changing magnetic field, which may couple to lines and induce undesired interferences. These interferences intensify with faster changing current amplitudes. That is, when switching the load circuit off, then the current dropping may affect undesired interferences, which intensify with the current decreasing faster. Accordingly the slew rate of the current when switching the load circuit off should be limited in order to lessen the produced magnetic field and thus to lessen interferences in adjacent lines.
Hence a transistor operated as a power switch should be protected from situations that heat its structure above an allowed temperature while at the same the slew rate of the discharge current should be limited in order to prevent undesired interferences in adjacent lines or electronic components. Particularly when switching a transistor off in an overload situation, i.e., in case the current through the transistor exceeds a predefined threshold value causing the transistor to be switched off, the main goal of controlling the switch-off process is to protect the transistor from being superheated.
However in deliberate switch-off situations, i.e., when the current through the transistor is below the predefined threshold indicating an overload situation, the process of switching the transistor to its off state can be optimized such that electromagnetic emissions are limited by controlling the slew rate while at the same time preserving the transistor from absorbing too much energy.